The Obama administration’s effort to overhaul antitrust rules for the meatpacking industry with the toughest regulations since the Packers and Stockyards Act was passed 90 years ago has run into strong opposition from Republicans.

The reforms, which would redraw the balance of power between meat companies and the farmers and ranchers who raise animals for them, have been one of the administration’s signature efforts in addressing the growing concentration of corporate power in agriculture. The new rules have faced resistance since they were proposed in June, but with Republicans in control of the House of Representatives after the November election, critics of the overhaul have powerful new allies, including the new chairman of the House Agriculture Committee.